Propylene, which is one example of lower olefins, has been known as a raw material for synthetic resin products such as polypropylene and acrylonitrile products, or for synthetic rubber products, but in recent years, its uses in the field of semiconductor electronic materials have been widespread. For these uses, propylene is required to have higher purity.
A raw material gas containing propylene as a main component, which is used as a raw material for attaining high purity, includes, for example, methane, ethane, propane, isobutane, and normal butane as impurities. As a method for purifying a propylene gas from the raw material gas, for example, various methods such as membrane separation, adsorption separation, and absorption separation have been known. Among these, methods of membrane separation, adsorption separation, and absorption separation, each using the adsorption characteristics of olefins to the silver ions, have been known.
In Patent Literature 1, a technique for separating and purifying olefins according to a method for membrane separation using silver ions is described. According to the technique described in Patent Literature 1, olefins are separated and purified from raw materials for olefins, containing paraffins as impurities, by using a separation membrane doped with silver ions.
In Patent Literature 2, a technique for separating and purifying olefins according to a method for adsorption separation using silver ions is described. According to the technique described in Patent Literature 2, olefins are separated and purified from raw materials for olefins, containing paraffins as impurities, by using an adsorbent having silver ions carried on alumina.
In Patent Literature 3, a technique for separating and purifying olefins according to a method for absorption separation using silver ions is described. According to the technique described in Patent Literature 3, olefins are separated and purified from raw materials for olefins, containing paraffins as impurities, by using an absorbing liquid containing silver ions.
Furthermore, it could also be found that main impurities included in olefins, which are separated and purified using silver ions, are paraffins and impurities remaining in high-purity olefins obtained by increasing the purity of the olefins are also paraffins. For example, the main impurities in propylene purified by the method described in Patent Literature 3 are propane (about 100 ppm by mole), and thus, high-purity propylene at about 99.99% by mole is obtained.
Meanwhile, propane which is one example of lower paraffins is used in the field of semiconductor electronic materials, for example, in the applications as raw materials for SiC, which is a next-generation power apparatus material, and is used as a propane gas diluted with hydrogen and as a pure gas of propane. For these uses, propane is required to have high purity.
A raw material gas containing propane as a main component, which is used as a raw material for high-purity propane, includes, for example, ethane, propylene, isobutane, and normal butane at high concentrations as impurities. Examples of a method for purifying propane from the raw material gas include methods of distillation, and membrane separation, adsorption separation, and absorption separation as described above for the purification of olefins. For example, in Patent Literature 4, separation of propylene and propane by a distillation process is described.